How can i edit a follow path constraint with the graph editor?
When you use a follow path constraint the movement and select Animate Path, there are no keyframes, the movement is determined by a couple of parameters:

1-The number of Frames on the path animation and Evaluation Time of the curve the object is parented to. The default is to move the object through the complete length of the path in 100 frames. 



2- A Generator Modifier in in the evaluation time that can be displayed the Graph Editor. The movement starting at 0 and increasing linerarly one unit in X (time) and one unit in (Evaluation time value)



So to determine the speed at which the object will travel through the path there are two options: 

A.- Change the number of frames on the curves path animation parameter. Less frames will make the object travel faster across the path. More frames will make it go slower.



B.- Altering the slope of the curve on the generator. A steeper line will make the movement faster, a flatter will slow it down.